Fiber optics transmission is the medium of choice for voice and data service provider networks. Currently, fiber optics provides the highest bandwidth per unit cost of any transmission medium. Early fiber optic systems, however, were expensive because they used proprietary architectures, equipment line codes, multiplexing formats, etc., such that one fiber optic system could not be connected to another unless they were both from the same manufacturer.
In response, Bellcore established a standard for connecting one optical fiber component or system to another. That standard is officially named the “Synchronous Optical Network” but is more commonly called “SONET.” The international version of the standard is officially named the “Synchronous Digital Hierarchy” but it is more commonly called “SDH.” Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both.
The basic SONET/SDH communications signal is defined as a Synchronous Transport Signal level 1 (STS-1). An STS-1 is comprised of so-called “SONET frames”, each being an 810-byte data packet comprising transport overhead (the information required to maintain communication) and payload (the data itself). For the purposes of this specification, a “STS-N” is defined to comprise N STS-1's. For example, an STS-768 comprises the data and overhead from 768 STS-1's. Furthermore, for the purposes of this specification, an “STS-N frame” is defined to comprise one byte per STS-1 SONET stream, i.e., a total of N bytes.
SONET/SDH provides specifications for digital transmission bandwidths that were previously not possible. By using equipment that can provide add/drop and through switching of SONET/SDH-based communications, commonly called “nodes”, service providers can use their currently embedded fiber optic transmission facilities to effect higher bandwidth over the same fiber. Further, SONET/SDH provides for a high level of communications signal integrity. SONET/SDH nodes can detect problems in transmission facilities and take corrective action, such as automatic protection switching.
To provide automatic protection switching, SONET/SDH specifies that two facilities be reserved through a SONET/SDH network: a working facility and a protection facility. When a problem is detected on the working facility (i.e., a discontinuity of signal or a high bit error rate), the nodes in the network switch from the working facility to the protection facility. This switch to the protection facility requires reconfiguration of a node's switching network and may include rearranging how signals are added to and/or dropped from the main transmission facility signal.
A problem in the art is that current switching networks nodes generally have multiple stages to provide non-blocking paths for maximum throughput. Generally, a switching network requires a “frame” of time to switch one STS-N frame through each stage. If a switching network is reconfigured immediately upon receipt of the automatic protection switching request, STS-N frames traversing the switching network are lost. Alternatively, STS-N frames already in the switching network complete all of the stages, while further STS-N frames are held up or dropped until the switching network is clear and the paths through the switching network are reconfigured for automatic protection switching. This time lag means that the communications takes a “hit”, because a portion of the communication signal is lost during the reconfiguration period.